It isn't clear if the act of labor induction or augmentation that resulted in the link, or the medications used in the procedures.

Babies born to women whose labor was induced, or whose contractions were strengthened, with medical procedures such as hormone treatments, face an increased risk of autism, a new study suggests.

Using school records and birth databases in North Carolina, researchers looked at the birth records of more than 625,000 children born between 1990 and 1998, including 5,500 who were diagnosed with autism.

They found that children born to mothers whose labor was induced, augmented or both had a 27-percent increased risk of autism, compared with children born to mothers whose labors were not induced or augmented.

The link held when the researchers took into account other factors that could have influenced the rate of autism, such as mother's age, diabetes during pregnancy and pre-term labor.

Still, other factors that were not accounted for could have lay beneath both the need for hormones during labor and a baby's risk of autism.

"While the results are interesting, we are not drawing a cause-and-effect relationship," said study researcher Simon Gregory, professor of medicine at Duke University in Durham, N.C.

"The risk to the mother and the unborn child would be far greater" if the labor were not induced or augmented when necessary, Gregory said.

Labor induction is a medical procedure that stimulates uterine contractions before labor begins on its own. A doctor may recommend this treatment when a pregnancy is more than a week or two past the due date, or when the health of the mother or her unborn child is at risk if the pregnancy were to continue. Labor augmentation, on the other hand, is a procedure that speeds up a labor that has already started, but is progressing too slowly.

It is not clear whether it is the act of labor induction or augmentation that underlies the link with autism, or the medications used in these methods, the researchers said. Moreover, it might be that certain medical conditions during pregnancy that can lead to the need for labor induction or augmentation are also responsible for the link with autism, the researchers said.

It is possible that exposure to the hormone oxytocin, which is used for labor induction, could explain the link, the researchers said. It is estimated that 50 to 70 percent of women who undergo labor induction receive oxytocin. Oxytocin might affect the baby's nervous system, perhaps in ways that depend on genetics, but this idea needs further study, the researchers said.

In the study, children born to mothers whose labor was induced (but not augmented) had a 13 percent increase in risk of autism, and children born after augmented labor had 16 percent increased risk of autism.

The results were particularly pronounced in male children, the researchers said. Boys born to mothers who received both induction and augmentation faced a 35 percent increase in autism risk, according to the study.

The study also confirmed links between autism and other previously suggested risk factors, such as pre-term birth and diabetes in the mother. Children born before week 34 of pregnancy were 25 percent more likely to have autism, compared with children born at full term. Mothers with diabetes — including both those with gestational diabetes and those who had diabetes prior to their pregnancies — had a 23 percent increased risk of having a child with autism, compared to mothers without diabetes.

The study had some limitations, the researchers said. The databases did not include information about every possible risk factor, and the researchers could not take into account the fathers' age, the medications the mothers might have used, or the severity of the children's autism.